Crumb rubber from scrap tires is used as an extender in rubber products. Tires for example use about 2-3% recycled rubber. However, inert crumb rubber, i.e., untreated vulcanized crumb rubber, significantly reduces the physical strength of rubber products. For this reason, inert crumb rubber is only used in products which also contain other polymers that cause the crumb rubber particles to adhere to one another. Furthermore, the products in which crumb rubber is typically used are those that do not encounter dynamic stress in use and do not require high tensile strength. Thus, crumb rubber is used mostly for such items as doormats, pads, and soaker hoses. If the adhesive polymer were not needed, molded objects could be made with up to 100% crumb rubber. Nevertheless, the processing of crumb rubber to make it suitable for use at such high levels is uneconomical due to high costs.
One process for using crumb rubber and increasing its strength involves combining 100 PHR of crumb rubber with 6 PHR of virgin rubber and 2-4 PHR of a mixture of plasticizer and softening chemical such as sulfides. (xe2x80x9cPHRxe2x80x9d denotes xe2x80x9cparts per hundred ratioxe2x80x9d, i.e., parts per 100 parts of devulcanized tire crumb rubber, on a volume basis.) The formulation is milled in a Banbury mixer or an extruder, then milled, rolled and aged to permit the sulfides to penetrate the rubber. Paraffin distillate has also been used as the softening agent. Solvents containing devulcanizing agents such as halogenated organics, organometallic complexes, acids, guanidines, and phenylamines, are mixed with crumb rubber to effect a surface devulcanization. These formulations and the methods of treating them entail costs of $0.15 to $0.42 per pound above the cost of the crumb rubber.
Another approach has been to treat the crumb rubber particles with chlorine gas. Crumb rubber treated in this manner has been used as an extender in elastomeric polyurethanes for such applications as industrial tires, wheels and automotive components.
Devulcanization of crumb rubber permits it to be used in a wider range of formulations and applications. When devulcanized elastomers are used, at least 70% devulcanization is generally considered necessary. Devulcanized recycled elastomers that are manufactured without chemical additives are preferred because they are easier to compound and to mold. The steam autoclave process can deliver unadulterated recycled rubber.
Another method that has been proposed for devulcanization is ultrasound. Ultrasound devulcanization of elastomers is a known process. As documented by Basedow and Ebert, in xe2x80x9cUltrasound Degradation of Polymers in Solution,xe2x80x9d Advances in Polymer Science 22: 88 (1977), the concept of using ultrasonic waves to break elastomeric chemical bonds of Cxe2x80x94C, Sxe2x80x94S, Sxe2x80x94Sxe2x80x94S, and Cxe2x80x94S has been known since the early 1950""s.
U.S. Pat. No. 3,725,314 (Pelofsky, Apr. 3, 1973) discloses immersing rubber in a liquid hydrocarbon and exposing the immersed rubber to ultrasonic energy at frequencies of from 10 to 40 kHz with a power intensity greater than 100 watts per square centimeter of material. By moving the material continuously past the ultrasonic energy source, hot spots are supposedly avoided. This method contemplates using a piezoelectric ultrasonic device and has not been commercialized, since no equipment is available for practicing this method on a commercial scale.
U.S. Pat. No. 4,548,771 (Senapati et al., Oct. 22, 1985) discloses an ultrasonic vulcanization process in which chemical bonds are formed in virgin elastomers at 200-300xc2x0 F. under a pressure of 500-100 psi, for periods up to ten minutes for each xe2x85x9-inch thickness of the material. The energy intensity is 20-200 watts per square inch, and the ultrasonic energy is 10-100 kHz. Like the method of the Pelofsky patent, this method contemplates using a piezoelectric ultrasonic device and has not been commercialized.
U.S. Pat. No. 4,104,205 (Novotny et al., Aug. 1, 1978) discloses a continuous process for breaking rubber bonds by microwave devulcanization in a continuously moving steel auger. The process was satisfactory on a laboratory scale but has never been commercialized.
U.S. Pat. No. 5,284,625 (Isayev et al., Feb. 8, 1994) discloses the placement of either single or multiple units of piezoelectric ultrasonic units in an extruder for continuous devulcanization of tire crumb rubber. A study in which this apparatus was used was reported by Mason, T. J., Ultrasonics 30(3): 192-196 (1992), who concluded that the apparatus would not work successfully due to hot spots and cavitation. Here as well, the limitations of piezoelectric ultrasound prohibit this method from being practiced economically on a commercial scale.
In the apparatus tested by Mason, an ultrasound horn with a piezoelectric transducer was placed within an extruder. When the device was scaled up by a harness that coupled several thousand piezoceramic ultrasound devices, the piezoceramic crystals disintegrated. This indicates that the apparatus disclosed in the Isayev et al. patent is only suitable for laboratory scale processes and is not feasible for commercial operations that require large throughputs. The disclosure in the Isayev et al. patent is similar to a disclosure of a uranium leaching unit in U.S. Pat. No. 4,071,225 (Holl, Jan. 31, 1978), by using seventy small 30-watt piezoelectric units to generate a total field of 2,000 watts. The effort described by Holl has never been successfully practiced as a commercial process.
It has now been discovered that ultrasonic vibrations induced by a magnetostriction transducer can be used successfully to achieve the dissociation of the carbon-sulfur and sulfur-sulfur crosslinking bonds of a vulcanized elastomer to achieve devulcanization. In a continuous flow process well adapted to the treatment of crumb rubber as the elastomer, the particulate crumb rubber is preferably passed through a conduit in which the ultrasonic vibrations are induced in directions perpendicular to the conduit axis, i.e., perpendicular to the direction of flow of the crumb rubber through the conduit. The directions of the vibrations are preferably radial relative to the conduit axis.